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Tire sizes and PSD

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Old 07-24-2007, 10:22 PM
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wino5150
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Default Tire sizes and PSD
I know lots of us run different tire and wheel combos, but how much can you vary from spec before the PSD light comes on and senses differences in wheel speed from front to back. The reason I ask this is that I have 17" track wheels and want to run 245/40 and 275/40. The front diameter will be smaller than the rear...problem? Thoughts?

Roland
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Old 07-24-2007, 10:57 PM
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I'm pretty sure the PSD only looks at left vs right. But ABS migth looks front / rear variation, but with the tires you want to run, I don't think you'll have a problem.
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Old 07-25-2007, 05:38 AM
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PSD and ABS both look for front back difference too. It doesn't matter what size tires are as long as all four are similar size. I would keep front back variation within 5% difference from stock.
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Old 07-25-2007, 09:37 AM
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Vilhuer is correct. 5% is about the limit, at least on newer systems.
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Old 07-25-2007, 09:46 AM
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Originally Posted by Vilhuer
PSD and ABS both look for front back difference too. It doesn't matter what size tires are as long as all four are similar size. I would keep front back variation within 5% difference from stock.
Why would a PSD system care about front/rear variance? It's a limited slip mechanism that's designed to measure drive traction variance (right vs. left). Since there is no drive at the front wheels, why would the front wheels come into play?
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Old 07-25-2007, 09:50 AM
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I just read the manual. You're right. It does measure front vs. rear, but it doesn't intuitively make sense why that would be. Must be an engineering thing.
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Old 07-25-2007, 09:51 AM
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What year does this start to become an issue?
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Old 07-25-2007, 09:58 AM
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PSD started in 1990 IIRC.
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Old 07-25-2007, 10:36 AM
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Originally Posted by Andrew Olson
I just read the manual. You're right. It does measure front vs. rear, but it doesn't intuitively make sense why that would be. Must be an engineering thing.
PSD brain is more advanced than just box that tries to keep both rear wheels rotating at same speed without looking for other things. It uses front wheel speeds to calculate steering angle. Angle and acceleration sensor under drivers seat is used to determine if car is sliding. Slide is then limited by locking or unlocking rear wheels as needed.

http://www.youtube.com/watch?v=_PoocIsiV2c
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Old 07-25-2007, 11:07 AM
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Neat video.
My '89 doesn't have that, but would running staggered sizes (255x50 & 225x50) mess up anything?
Not a lot of choices in 16's and thought a set of RA1's might be nice on the factory wheels sometimes.
Thanks.
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Old 07-25-2007, 11:28 AM
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http://www.miata.net/garage/tirecalc.html

Code:
Specification  Sidewall  Radius  Diameter  Circumference  Revs/Mile  Difference
225/50-16         4.4in  12.4in    24.9in         78.1in        811        0.0%
255/50-16         5.0in  13.0in    26.0in         81.8in        775        4.8%
Its right at the limit and could cause ABS system to raise error.
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